In general, a unit pixel of a complementary metal oxide semiconductor (CMOS) image sensor includes a photodiode and a metal oxide semiconductor (MOS) transistor. The CMOS image sensor sequentially detects an electrical signal of each unit pixel in a switching manner to generate an image.
CMOS image sensors are classified into 3T-type, 4T-type, and 5T-type CMOS image sensors according to the number of transistors in each unit pixel. For example, a unit pixel of the 4T-type CMOS image sensor includes a photodiode PD, a transfer transistor Tx, a reset transistor Rx, a drive transistor Dx, and a select transistor Sx. The photodiode PD receives light to generate photoelectric charges. The transfer transistor Tx transfers photoelectric charges generated in the photodiode PD to a floating diffusion region FD. The reset transistor Rx sets an electric potential of the floating diffusion region FD to a desired level and resets the floating diffusion region FD by discharging photoelectric charges. The drive transistor Dx serves as a source follower buffer amplifier, in which a voltage of the floating diffusion region FD is applied to a gate. The select transistor Sx performs an addressing through a switching operation.
In such a CMOS image sensor, light is incident to a photodiode to generate an electron-hole pair in the photodiode and the electron-hole pair is moved to a floating diffusion region.
Here, a defect such as a dangling bond may occur on the surface of a semiconductor substrate. The dangling bond traps electric charges to cause noise when the electric charges moves from the photodiode to the floating diffusion region.